Name:____________________________________ Class Period:________ Date:_____________
Chemistry Laboratory Experiment
Investigating Evaporation and Intermolecular Forces
Introduction
Evaporation is an endothermic process. Energy is required to overcome the intermolecular
forces (IMF) between the liquid molecules so they can escape into the vapor state. The
stronger the intermolecular forces, the more energy is required. The energy for evaporation
comes from the surroundings, causing a cooling effect. This is how perspiring
helps you cool – water on your forehead evaporates cooling your forehead in
the process. Animals cool off in a variety of ways (pigs roll in mud, dogs pant,
etc.) but each method depends in part upon the endothermic process of water
evaporation. The rate of evaporation is inversely proportional to the strength of
the intermolecular forces. In other words, the rate of evaporating will be faster
with a substance that has weak intermolecular forces. A fast rate of evaporation will result in a
larger temperature drop than a slow rate of evaporation for equal volumes of liquid. Have you
ever had your arm wiped with rubbing alcohol before a shot? It feels cooler than water
evaporation because the alcohol has weaker intermolecular forces and evaporates faster than
water.
Recall that the strongest type of intermolecular force is hydrogen bonding,
which occurs between molecules that have a hydrogen atom bonded to a
small electronegative atom like oxygen, nitrogen or fluorine. Dipoledipole forces occur between the positive end of a polar molecule and the
negative end of another polar molecule. All molecules posses London
dispersion forces, which are the result of the electron cloud of a molecule becoming
instantaneously polarized and inducing a polarized electron cloud of a neighboring molecule.
The more massive the molecule the larger the electron cloud and the more polarizable it is,
leading to greater London dispersion forces. This is the only type of intermolecular force
between nonpolar molecules. In general, polar molecules have stronger intermolecular forces
than nonpolar molecules.
In this laboratory experiment you will predict and measure the cooling
effect of different liquids. Your knowledge of Lewis structures, VSEPR theory, molecular
geometry and intermolecular forces will allow you to rate the polarity of the liquids. The type of
IMF, along with molar mass, will allow you to predict the relative cooling effect of the different
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
liquids. Then you will test your prediction by measuring a temperature drop during
evaporation.
Pre-lab
For each of the liquids in the table below, draw the structure. Use WebMO to confirm the
Lewis structure that you drew. Determine the molar mass and indicate whether the molecule is
polar or nonpolar and state the type of intermolecular forces that are present.
Liquid
Formula
Methanol
CH3OH
Ethanol
CH3CH2OH
n-propanol
CH3CH2CH2OH
n-butanol
CH3CH2 CH2CH2OH
n-pentane
CH3CH2 CH2CH2CH3
Molar
Mass
g/mol
Lewis
Structure
Polar or
Predominate
Nonpolar Type of IMF
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
Liquid
Formula
Hexanes
CH3 (CH2 )4CH3
Heptanes
CH3 (CH2 )5CH3
Octane
CH3(CH2)6 CH3
Glycerine
OHCH2CH(OH)CH2OH
Water
HOH
Acetone
CH3COCH3
Molar
Mass
g/mol
Lewis
Structure
Polar or
Type of IMF
Nonpolar
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
Underneath the liquid name, indicate the order of rate of evaporation that you expect (1 =
fastest and 11 = slowest) and the dipole moment (D) from WebMO.
Materials
Filter paper cut into 2.5 x 2.5 cm pieces
Vernier temperature probe, interface, Logger Pro and computer or
Digital thermometer and stopwatch or glass thermometer and stopwatch
Small rubber bands (tubing cut in thin sections)
Masking tape
Test tubes with stoppers
Test tube holder
Part A - Effect of Molar Mass - Liquids: methanol, ethanol, n-propanol, n-butanol
Part B – Effect of Molar Mass – Liquids: pentane, hexanes, heptanes, octane
Part C – Effect of Number of Hydrogen Bonds – Liquids: glycerin , water, methanol, hexane
Part D – Effect of Type of Intermolecular Force – Liquids: water, n-propanol, acetone, hexanes
Safety
Caution! The liquids used in this experiment are flammable. There should be no open
flames or sources of sparks near the liquids.
Avoid inhaling the vapors or contact with your skin or clothing.
Let your teacher know immediately if a spill or accident occurs.
Obtain and wear safety glasses and lab aprons.
Procedure
Your teacher will assign the parts of this experiment that you will be doing. If you will not be
doing all parts, then the data obtained from each group will be shared. The procedure for each
liquid of each part is identical and is given below. With care, two liquids can be done at once.
1. Obtain stoppered test tubes containing about 3 mL each of the liquids that you will
measure. Make sure that they are marked so that you can identify them.
2. Obtain as many pieces of 2.5 cm x 2.5 cm filter paper as liquids that you will test.
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
3. Obtain or cut 2 small rubber bands and 2 pieces of masking tape.
4. Secure the filter paper to the probe or thermometer. Make sure to only cover the top
portion of the filter paper so as to allow for evaporation.
5. Place the wrapped probe or thermometer in the test tube with the liquid. After one
minute record the initial temperature as the “time zero” temperature in the data table.
6. Quickly remove the probe or thermometer and place it on the lab bench so that the
filter paper end hangs over the edge. Secure in place with the masking tape.
7. Record the temperature every 10 seconds until the temperature reaches a minimum
and then begins to increase again or for 5 minutes, whichever comes first. Make sure to
avoid breezes and do not blow on the filter paper to make it evaporate faster.
8. Repeat this process with the next liquid.
9. Clean up according to your teachers instructions.
10. Obtain and share data with your classmates as directed by your teacher.
Calculation Table
Record the molar mass and temperature change for each liquid.
Liquid
Temperature change =
tinitial-tminimum (Co)
Molar Mass
(g/mol)
Methanol (A, C)
Ethanol (A)
Propanol (A, D)
Butanol (A)
Pentane (B)
Hexane (B, C, D)
Heptanes (B)
Octane (B)
Glycerine (C)
Water (C, D)
Acetone (D)
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
Analysis of Data
1. Make a graph of temperature change vs. molar mass for the “A” solvents. Connect the
data points with a smooth curve. To the graph add the “B” solvents. Make sure to
include a key with your graph. (This is done most easily using Excel.)
2. Make a graph of the change in temperature vs. the number of hydrogen bonds for the
“C” solvents.
Questions
1. From the graph for the “A” and “B” liquids, how did molar mass affect the rate of
evaporation? Remember a small temperature difference indicates a slow rate of
evaporation.
2. From the graph of liquids “C” what can you conclude about the number of hydrogen
bonds and the rate of evaporation?
3. What is the main difference between liquids “A” and liquids “B”?
4. For liquids “D” was the most polar solvent the one with the slowest evaporation rate?
Was the least polar compound the one with the fastest evaporation rate? Explain your
answer. Include discussion of the dipole moments obtained from WebMO and the type
of intermolecular forces.
5. Based on your data, what do you think has a greater effect – molar mass or type of
intermolecular force? Explain and justify your answer.
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
6. Did the order of evaporation rate that you indicated in the pre-lab exercise, correspond
with your results. Please indicate any differences and offer an explanation for the
discrepancy.
7. What are some sources of error in this lab?
Data Table
“A” Liquid
Methanol
Ethanol
Propanol
Butanol
Time (s)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
170
180
190
200
210
220
230
240
250
260
270
280
290
300
Data Table
“B” Liquid
Pentane
Hexane
Heptane
Octane
Time (s)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
Data Table
“C” Liquid
Glycerine
Water
Propanol
Hexane
Time (s)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
0
10
20
30
40
50
60
70
80
90
100
110
120
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
Data Table
“D” Liquid
Water
Acetone
Propanol
Hexane
Time (s)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
Temperature
(oC)
0
10
20
30
40
50
60
70
80
90
100
110
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
Name:____________________________________ Class Period:________ Date:_____________
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
Adapted from Flinn Experiment 9: Evaporation and Intermolecular Attractions; Chemistry with Computers, 2 nd
Edition; 2000.
ICLCS Team 4: The 3-Ds
Jeanne M. Capriotti ; Bradley-Bourbonnais Community High School; 2008
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright